服务调用几乎始终是双向操作，这意味着请求是从使用者发送到提供者的，而响应是按照相反的方向返回的。
A service invocation is almost always a two-way operation, meaning that a request is sent from the consumer to the provider and a response is sent back in the other direction.
代理流程包含一个接收活动，后面跟着一个调用活动，以及一个仅当该流程提供的操作是双向操作时才需要的应答活动。
The process contains a receive activity followed by an invoke activity, and a reply activity that is needed only if the operation offered by the process is a two-way operation.
如果被调用的操作是单向的，则不存在fault或out末端，因为这两种末端只能存在于双向操作中(请参见图6)。
If the operation being invoked is one way, there's no fault or out terminals because these can only exist for two-way operations (see Figure 6).
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